Improve the readibility of the function which inserts command buffers
and other maintanence commands into the GPUs ring buffer.  We do this
by splitting the ring buffer reservation in two: one chunk for any
commands that need to be issued prior to the command buffer, and a
separate chunk for commands issued after the buffer.

The result is a much more obvious code flow in this function, and
localisation of the conditional maintanence commands prior to the
command buffer.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>